Alzheimer's disease (AD) is characterised by worsening cognitive impairment, affecting memory, that debilitates the patient's social and occupational functioning. The degenerative disease causes loss of nerve cells within the brain, which brings about cognitive difficulties with language and higher functioning, such as judgement, planning, organisation and reasoning, which can lead eventually to personality changes. The end stages of the disease are characterised by a complete loss of independent functioning.
Histologically, AD (sporadic and familial) is defined by the presence of intracellular neurofibrillary tangles (NFT's) and extracellular plaques. Plaques are aggregations of amyloid β peptide (Aβ) derived from the aberrant cleavage of the amyloid precursor protein (APP), a transmembrane protein found in neurons and astrocytes in the brain. Aβ deposits are also found in the blood vessels of AD patients.
Cholinergic neurons are particularly vulnerable in AD, and the consequent neurotransmitter decline affects other neurotransmitter systems. Other symptoms of the disease include oxidative stress, inflammation and neuronal apoptosis (programmed cell death). In the AD patient, extensive neuronal cell death leads to cognitive decline and the eventual death of the patient. (Younkin, 1995; Borchelt et al., 1996; Selkoe, 1999).
Current treatments are symptomatic only and are seen as minimally effective with minor improvements in symptoms for a limited duration of time. However, overproduction or changes in Aβ levels are believed to be key events in the pathogenesis of sporadic and early onset AD. For this reason, Aβ has become a major target for the development of drugs designed to reduce its formation (Vassar et al., 1999), or to activate mechanisms that accelerate its clearance from brain.
The amyloid cascade hypothesis proposes that production of the Aβ peptide adversely affects neuron function, thereby, leading neuron death and dementia in AD. Aβ is produced from the amyloid precursor protein (APP) which is cleaved sequentially by secretases to generate species of different lengths. The main plaque component is the 42 amino acid isoform of Aβ1-42 which is involved in the formation of neurotoxic oligomers and plaque formation in AD pathogenesis. A number of isoforms of Aβ including Aβ1-42, pGluAβ3-42, Aβ3-42 and 4-42 predominate in the AD brain, of which Aβ1-42 and Aβ4-42 are the main forms in the hippocampus and cortex of familial and sporadic AD (Portelius et al., 2010).
Aβ ending at residue 42 is a minor component of the Aβ species produced by processing of APP. Other forms include Aβ1-40 and N-terminal truncates Aβn-40. However, Aβ ending at residue 42 is most prone to aggregate and drives the deposition into amyloid plaques. In addition to being more prone to aggregate, the Aβ1-42 peptide forms soluble low-n polymers (or oligomers) that have been shown to be toxic to neurons in culture. Unlike the larger conspicuous fibril deposits, oligomers are not detected in typical pathology assays. Oligomers having similar properties have been isolated from AD brains and these are more closely associated to disease progression than the plaques (Younkin, 1998; Walsh et al., 2005a; Walsh et al., 2005b).
Experimentally generated oligomers applied to brain slices or injected in vivo cause failure of hippocampal long-term potentiation (LTP) which is a form of synaptic information storage well known as a paradigm for memory mechanisms (Lambert et al., 1998; Walsh et al., 2002; Wang et al., 2002). Soluble oligomers have been involved in the physical degeneration of synapses (Mucke et al., 2000). Reversal of memory failure by antibodies in mouse models has confirmed the emerging concept that oligomers have a major role to play in synaptic failure.
Genetic evidence suggests that increased amounts of Aβ1-42 and N-terminal truncates thereof (Aβn-42) are produced in many, if not all, genetic conditions that cause familial AD (Borchelt et al., 1996; Duff et al., 1996; Scheuner et at, 1996; Citron et al., 1998), pointing to the possibility that amyloid formation may be caused either by increased generation of Aβn-42 or decreased degradation, or both (Glabe, 2000). In particular, familial AD causing genetic mutations in the APP gene and/or in the gene encoding the γ-secretase complex component presenilin increased the production of Aβ1-42 relative to Aβ1-40. It has also been proposed that the absolute quantity of peptides produced within the brain might be less important than the ratio of Aβ peptides (reflected in a changed Aβ1-42 to Aβ1-40 ratio) for the generation of toxic Aβ species (De Strooper, 2007; Kuperstein et al., 2010). In addition, animal models of amyloid deposition, both mice and Drosophila, suggest that Aβ1-42 is required for the formation of amyloid deposits (Greeve et al., 2004; Iijima et al., 2004; McGowan et al., 2005).
Results from a vaccination study in 2000 suggested possible new treatment strategies for AD. The PDAPP transgenic mouse, which overexpresses mutant human APP (in which the amino acid at position 717 is phenylalanine instead of the normal valine), progressively develops many of the neuropathological hallmarks of AD in an age- and brain region-dependent manner. Transgenic animals were immunised with Aβ1-42 peptide either before the onset of AD-type neuropathologies (at 6 weeks of age) or at an older age (11 months), when Aβ deposition and several of the subsequent neuropathological changes were well established. Immunisation of the young animals essentially prevented the development of plaque formation, neuritic dystrophy and astrogliosis. Treatment of the older animals also markedly reduced the extent and progression of these AD-like neuropathologies. It was shown that Aβ1-42 immunisation resulted in the generation of anti-Aβ antibodies and that Aβ-immunoreactive monocytic/microglial cells appear in the region of remaining plaques (Schenk et al., 1999; Schenk et al., 2000). However, the active immunisation approach when applied to humans resulted in several cases of meningoencephalitis, most likely due to a T-cell response, and was discontinued although the initial results on efficacy were promising (Orgogozo et al., 2003; Gilman et al., 2005; Pride et al., 2008).
Following this, several passive vaccination strategies were investigated. The peripheral administration of antibodies against Aβ was sufficient to reduce amyloid burden (Bard et al., 2000). Despite relatively modest antibody serum levels achieved in these experiments, the passively administered antibodies were able to cross the blood-brain barrier and enter the central nervous system, decorate plaques and induce clearance of pre-existing amyloid. In a comparison between an Aβ1-40-specific antibody, an Aβ1-42-specific antibody and an antibody directed against residues 1-16 of Aβ, all antibodies were shown to reduce Aβ accumulation in mouse brain (Levites et al., 2006).
More recently, it has been suggested that CNS penetration is the most likely route to effective Aβ clearance for passively administered antibodies (Golde et al., 2009). However, in addition to the antibodies being able to cross the blood-brain barrier, the sink hypothesis was proposed as a possible mechanism of action.
The sink hypothesis states that Aβ can be removed from CNS indirectly by lowering the concentration of the peptide in the plasma. In the experiments describing this, an antibody that binds the Aβ in the plasma and thereby sequesters Aβ from the CNS was used. This was accomplished because the antibody prevents influx of Aβ from the plasma to CNS and/or changes the equilibrium between the plasma and CNS due to a lowering of the free Aβ concentration in plasma (DeMattos et al., 2001). Amyloid binding agents unrelated to antibodies have also been shown to be effective in removing Aβ from CNS through binding in plasma. Two Aβ binding agents, gelsolin and GM1, which sequester plasma Aβ were shown to reduce or prevent brain amyloidosis (Matsuoka et al., 2003).
Regarding safety, one pathogenic feature in AD is cerebral amyloid angiopathy (CAA) where there is a replacement of vascular smooth muscle cells with Aβ, mainly Aβ1-40, in the walls of cerebral arteries (Weller et al., 2003). Treating AD patients with pan-Aβ antibodies has been shown to lead to microhemorrhages reflecting the removal of Aβ from the vessel wall (Wilcock et al., 2009) which could be detrimental to patients. One way to circumvent this has been to generate de-glycosylated antibodies which may reduce the clearance mechanisms contributing to microhemorrhages and/or reduce the rate by which Aβ is cleared from the vascular deposits, preventing saturation of efflux pathways (Wilcock et al., 2006).
Targeting the n-42β peptide species with an Aβ42 specific antibody would target the species which is the key peptide composite in the AD brain and the driver of plaque formation. An antibody with a primary specificity for n-42 monomer and low n oligomer species would not only deplete these species, but could also prevent the build-up of other oligomeric species shown to be toxic to neurons.